1. Field of the Invention
The present invention relates generally to the separation of complex organic mixtures into various components and, more particularly, to processes of separating low molecular weight fractions from complex organic mixtures utilizing supercritical solutions and is a continuation of U.S. application Ser. No. 154,210 filed on Feb. 10, 1988, now abandoned. Specifically, the present invention relates to the supercritical separation of low molecular weight components from complex organic mixtures utilizing supercritical carbon dioxide modified by the presence of an entrainer.
2. Description of the Prior Art
There are a wide variety of industrial biomass processing systems which produce waste streams containing substantial portions of useful constituents. Examples of such industrial systems include those involved in the manufacturing of pulp and paper such as black liquor solutions in kraft processes, in the production of cheese and whey, and in other biomass processing systems. The annual volume of such waste processing streams is very substantial. Thus, there have been numerous efforts over the years to separate useful components from such waste streams for the purpose of recirculation within the processing system, for the separate sale and/or use of such components, or for environmental purposes to remove environmentally damaging components from the waste streams. Due to the volume involved in such industrial systems, economic factors such as the complexity of the separation process or the energy requirement for such separation processes become extremely important as compared to the efficiency of the separation process as well as the effectiveness in terms of extraction capability.
For example, the kraft process of converting wood into cellulose pulp includes treating the lignocellulosic material with sodium hydroxide/sulfide solutions. During this process, lignins are dissolved and hemicelluloses are degraded to a complex mixture of organic compounds including various carboxylic acids such as saccharinic acids. Low molecular weight components such as phenolic compounds derived from lignins are present in streams of the black liquor or in washing operations such as preparation of chemically pure cellulose by dissolution of pulp in alkaline solutions.
The separation of large molecular weight components is relatively straightforward, and many prior art techniques have been developed to separate such high molecular weight components from the complex organic mixtures. Some techniques have also been developed for the separation of the low molecular weight fractions. These prior art techniques contain many stages involving ion exchange, adsorption steps, water evaporation, distillation, and other purification operations. These standard and well known procedures are very complex and expensive to operate. In order to increase the efficiency of separation, use of supercritical fluids to enhance separation has been developed. U.S. Pat. No. 3,969,196 is an example wherein a number of organic compounds are separated utilizing a wide variety of supercritical fluids including carbon dioxide. While carbon dioxide is a desirable supercritical fluid due to its relative availability and inexpensiveness, this particular reference was unable to separate the more complex polyhydroxy compounds and other complex phenolic low molecular weight compounds utilizing carbon dioxide and, instead, had to utilize different supercritical fluids having differing solvation characteristics. These supercritical fluids are more complex to handle and more expensive than supercritical carbon dioxide.
Other references which disclose the use of carbon dioxide in separation processes include U.S. Pats. No. 2,772,965, No. 4,349,415, No. 4,437,939 and No. 4,474,994. None of these references illustrate the use of supercritical carbon dioxide to extract the more complex phenolic low molecular weight constituents from complex organic mixtures, nor do they illustrate the modification of the carbon dioxide solvating power by the addition of entrainers. Thus, these known processes do not address the effective separation of simple and complex low molecular weight constituents of complex organic mixtures (phenolic, complex hydroxyacids) from biomass processing systems utilizing inexpensive supercritical fluids.
U.S. Pats. No. 2,631,966, No. 2,632,030 and No. 2,698,278 all illustrate the use of liquid carbon dioxide with other co-solvents for separation purposes. However, these patents are limited to petroleum refining and do not deal with carbon dioxide at supercritical conditions, that is supercritical temperature and pressure conditions. Moreover, the compounds present in oil stocks for petroleum refining have very little in common with separation processes of complex organic mixtures derived from biomass processing systems as discussed above.
Thus, there remains a need for relatively simple processes of extracting useful low molecular constituents from complex organic mixtures derived from biomass processing streams. For such processes to be economically effective, they must preferably utilize mild temperature conditions and intermediate pressure ranges, they must be simple, and they must utilize relatively inexpensive chemical components as well as simplified hardware.